Collagen, the most abundant fibrous protein in the body, plays a major role in the structural stability of many tissues within the body (skin, tendons, cardiovascular system, cartilage, basement membranes, etc.), which make it a primary target for use in restorative medicine and other tissue-engineering applications for many different tissue types. Currently, harvested collagen that is primarily taken from bovine, porcine and human sources is used in many different clinical applications including cosmetic surgery, joint repair, artificial skin grafts, vascular tissue regeneration and even as a drug-delivery carrier. These methods follow multiple avenues for sample preparation ranging from the insertion of reconstituted collagen without the addition of any cells or growth factors into the body to the mixing of collagen with other polypeptides or polymers to form a scaffold upon which cells, growth factors and other biocompatible molecules are loaded before the complex system is implanted into the human body. And commercially available products, such as, Chondro-Gide, TransCyte, Apligraf, Integra and Matrigel demonstrate the current clinical and commercial interest in collagen-based systems.
Collagen exemplifies multi-hierarchical self-assembly. In the case of type I collagen, self-assembly begins with three 1,000 amino acid peptide strands which adopt a poly-proline type II helical structure and wind around one another forming a superhelical trimer. This structure defines the well-known collagen triple helix. These triple helices then pack against one another in quasi-hexagonal and staggered fashion, forming a nanofibrous structure known as collagen fibrils.1,2 Collagen fibrils continue to self-assemble both linearly and laterally forming collagen fibers and a hydrogel network (FIG. 1a). Together, the multiple levels of collagen's structural hierarchy play a major role in the structural integrity of the extracellular matrix and provide binding sites for other proteins and cells.
Collagen has been the target of biomimetic design for decades due to the fact that there are many difficulties associated with the use and characterization of collagen from natural sources and by expression. The use of recombinant systems requires either genetic modifications or the use of a novel biosynthetic pathway in E. coli in order to express hydroxyproline-containing collagens.3-8 Many successes have been demonstrated in the recapitulation of the collagen triple helix in short peptides, both as a homotrimer9-17 and, more recently, as a heterotrimer.18-24 However, examples that take these collagen-like peptides and use them to mimic the higher order assembly of collagen have faced a great deal of difficulty: in all previously reported systems, none has discretely demonstrated each level of collagen self-assembly (triple helix, nanofiber and hydrogel) within the same system. There are many examples of peptides that form organized nanostructures without gelation,10,12,25-29 and a few have achieved gelation without proving the presence of triple helices or nanofibers,30,31 however no system has described triple helix formation, nanofiber formation and gelation.
One example of a fiber forming collagen-like peptide was demonstrated by Chaikof and Conticello who prepared a 36 amino acid peptide with the sequence (Pro-Arg-Gly)4(Pro-Hyp-Gly)4(Glu-Hyp-Gly)4.32 This zwitterionic peptide was found to assemble into large organized fibers. However, even these collagen mimetic fibers have some drawbacks which include 1) a mixed composition of fibers associated with a significant quantity of other amorphous material, 2) the requirement for specific concentration and buffer composition outside of which the quality of assembly degrades or fails all together and 3) phase separation and precipitation of the formed fibers as opposed to the formation of a hydrogel.32 
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While the present disclosure is susceptible to various modifications and alternative forms, specific example embodiments have been shown in the figures and are herein described in more detail. It should be understood, however, that the description of specific example embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, this disclosure is to cover all modifications and equivalents as illustrated, in part, by the appended claims.